C. elegans and other nematodes can halt development under appropriate conditions (overcrowding, reduction in food supply) by becoming a nonfeeding stage (an alternative third larval stage) called the dauer larva. This stage is similar to the dispersal stage of many parasitic nematodes (Roberts and Janovy, Jr., 1996). C. elegans dauers can exist for extended periods (at least two months) without effects on subsequent post-dauer life-span or progeny production (Klass and Hirsh, 1976).
Several genes affect the transition into and out of the dauer state (Riddle et al., 1981). These dauer formation (daf) genes can be mutated to cause animals to always (the Daf-C, dauer constitutive, phenotype) or never (the Daf-D, dauer defective, phenotype) form dauer larvae. Most researchers agree that two interlinked genetic pathways control dauer formation (Vowels and Thomas, 1992; Thomas et al., 1993, Gottlieb and Ruvkun, 1994; Larsen et al., 1995).
Recent work has implicated one of these pathways in the control of aging. Specifically, Kenyon et al. (1993) showed that a daf-c mutation in the daf-2 gene caused a doubling of life-span in C. elegans. This increase was suppressed by a daf-d mutation in the daf-16 gene. Larsen et al. (1995) extended these observations, finding that daf-c mutations in daf-2 and daf-23 extended life-span. This extension of life-span was suppressed completely by mutations in daf-16 and partially by mutations in daf-18. These workers also found that certain daf-2; daf-12 double mutants had greatly extended life-spans, even though daf-12 mutations on their own did not affect aging.
These results place the most famous C. elegans aging gene, age-1, in a broader context. An age-1 mutation was identified by Klass (1983) and studied by Friedman and Johnson (1987), who showed that it extended both average and maximum life-span in C. elegans. The age-1 mutation is now known to be an allele of daf-23 (Malone et al., 1996). Moreover, Morris et al. (1996) have found that daf-23 encodes a subunit of a PI 3-kinase, indicating a role of signaling in determinating life-span.
The dauer pathway, when expressed in adults, allows animals to survive for relatively long periods of time (at least a four-fold extension in mean life-span). The targets of the dauer pathway genes that allow life-span extension, however, have not been identified, previously. Some of the ultimate targets for the daf genes may be genes encoding antioxidant enzymes, since catalase and superoxide dismutase (SOD) activities are approximately five times higher in the dauer larvae than in L3 worms (Anderson, 1982; Larsen, 1993). Moreover, unlike the activities in wild-type animals, total SOD and catalase activities increase with age in age-1 mutants (Vanfleteren, 1993; Larsen, 1993).
Several investigators have hypothesized that oxidative damage to cells is a major cause of cellular and organismal senescence (Gershmann et al., 1954; Harman, 1956; Sohal and Allen, 1990). Most relevant to our studies are experiments in Drosophila melanogaster where overexpression of Cu/Zn SOD and catalase, but not either alone, increased mean adult life-span by 33% (Seta et al., 1990; Stavely et al., 1990; Orr and Sohal, 1992, 1993, 1994). These results not only suggest that control of reactive oxygen species is an important determinant of longevity, but also underline the need to balance SOD and catalase activities for the control of oxidative stress (see also Phillips et al., 1989 and Amstad et al., 1991).
Here we show that C. elegans contains two catalase genes. One gene, ctl-1, appears to be needed for normal life-span and for the extension of life-span seen in daf-c adults. One striking feature of the ctl-1 catalase is its localization in the cytosol, not in peroxisomes. This localization is unusual, since cytosolic catalases have rarely, if ever, been seen in animals. The second C. elegans catalase gene, ctl-2, appears to encode the peroxisomal catalase. We suggest that the ctl-1 catalase is needed during periods of starvation, such as the dauer larva, and that its expression in daf-c adults enables them to live longer. As such ctl-1 would represent a true life-span extension gene.
In nematodes an alternative third larval stage, often called the dauer stage in free-living animals, allows animals to weather periods of low food availability (if free living) or to disperse (if parasitic). Mutations in several genes that control entry into and exit from the dauer stage of the nematode Caenorhabditis elegans profoundly affect the life-span of adults. The ctl-1 gene, which encodes an unusual, cytosolic catalase, is required in C. elegans for the life-span extension exhibited by animals with these dauer mutations. Cytosolic catalase may have evolved in nematodes to allow prolonged periods of dormancy before reproductive maturity.